error: cannot find attribute `unknown` in this scope
  --> $DIR/cfg-attr-unknown-attribute-macro-expansion.rs:3:27
   |
LL |         #[cfg_attr(all(), unknown)]
   |                           ^^^^^^^
...
LL | foo!();
   | ------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error